1. Field of the Invention
The present invention is generally related to insulation panels, and more specifically to vacuum insulation panels that have a high degree of resistance to heat transfer and are thin and bendable to form curved insulation panels, applications of such panels, and methods of making same.
2. Description of the Prior Art
There is nothing new about insulation or in the use of vacuum panels for insulation. There is, however, a rapidly emerging need for much improved insulation in terms of a combination of better insulation effectiveness, lighter weight, thinner, more durable, and more bendable or formable insulation products. The needs for such better insulation products emanate from such diverse areas as space-related vehicles and equipment, extremely low-temperature cryogenic vessels and pipes in scientific and industrial applications, and even common household appliances. For example, space vehicles and equipment to be launched into space need a very high quality of insulation to protect humans and equipment, yet there is no room for typically bulky insulated walls and panels.
State-of-the-art insulation for cryogenic applications is complex and expensive and still has significant shortcomings. For example, an insulation structure known as "cryopumped insulation" is often used for insulating cryogenic vessels and pipes. Such cryopumped insulation comprises many laminated layers of impervious material sealed at the edges and positioned adjacent the cryogenic material; e.g., liquid nitrogen. The liquid nitrogen is so cold that it causes the air in adjacent sealed spaces between the laminated sheets to liquify, thus leaving a partial vacuum in the spaces. This air-liquifying phenomenon occurs through adjacent layers a sufficient depth into the laminated insulation structure such that heat transfer is inhibited by the adjacent vacuum layers created or "cryopumped" in the insulation structures.
While such "cryopumped" insulation works quite well at the extremely low temperatures of cryogenic materials, like liquid nitrogen, which are cold enough to liquify air, it does not insulate at all in normal temperature ranges. Also, such "cryopumped" insulation is relatively thick and bulky, typically requiring several inches of thickness to be effective, and it is expensive and difficult to form into desired shapes or contours. Yet, prior to this invention, there really were no thin, non-bulky, formable, yet equally effective alternatives.
On the domestic scene, both consumers and governments are demanding that manufacturer of home appliances, such as refrigerators, water heaters, dishwashers, washing machines, clothes dryers, and the like, make these appliances much more energy efficient. For example, the California Energy Commission has mandated a 50% reduction in the energy use of refrigerator/freezers to be sold in that state in 1992. That mandated reduction in energy usage, while maintaining current dimensions, is not achievable without significant improvement in side-wall thermal efficiencies. Current technology could accommodate the reduction in heat transfer through the side walls of appliances by making insulated wall panels three times as thick. However, people do not want refrigerators with walls a foot thick. Architectural designs of homes and apartments, door widths, and the like practically prohibit increasing external dimensions of home appliances, and decreasing useable interior space will meet with much consumer dissatisfaction and resistance.
Thinner insulation panels that improve insulating effectiveness would solve these problems, but ultra-thin, highly effective, and long-lasting insulation panels are not easy to make. In fact, prior to this invention, each of these criteria, i.e., ultra-thin, highly effective, and long-lasting, has been mutually exclusive of at least one of the others.
There have been some notable attempts prior to this invention to improve insulation effectiveness with somewhat thinner panels. For example, U.S. Pat. No. 2,989,156, issued to F. Brooks et al., discloses an insulation panel comprising an evacuated space between metal sheets, which evacuated space is filled with perlite powder. U.S. Pat. No. 3,151,365, issued to P. Glaser et. al., shows the use of a mixture of fine carbon black particles and other fine particles filling an evacuated, enclosed structure, intermediate foil radiation shields, and an emissivity-reducing coating of silver. The H. Strong et al., patent, U.S. Pat. No. 3,179,549, uses a mat of very fine, oriented glass fibers sealed inside an evacuated, welded metal envelope. The vacuum used is only about 10.sup.-4 atmospheres (10.sup.-2 Torr), and it requires a fiber mat of sufficient density and thickness to be opaque to thermal infrared radiation. U.S. Pat. No. 4,444,821, issued to J. Young et al., also discloses an evacuated panel filled with a glass fiber mat with plastic edge seal strips and a gettering material positioned in the evacuated chamber. This panel also specifies only a low-grade vacuum of about 10.sup.-2 Torr. The N. Kobayashi patent, U.S. Pat. No. 4,486,482, also uses a glass fiber mat inside a vacuum envelope made of welded stainless steel sheets. This glass mat is stitched with glass fibers that run perpendicular to the plane of the mat and are supposed to support the external atmospheric pressure load on the panel walls to keep them from collapsing.
The above-described prior art vacuum panels are no doubt more effective than conventional foam and fiberglass insulation panels. However, constructing a truly effective and long-lasting insulation panel is not easy and is not achieved by these prior art structures to the extent necessary to meet the needs described above. For example, the low-grade vacuums used in the prior art patents cannot achieve insulation efficiencies high enough for use in ultra-thin panels. Plastic edge seals cannot maintain a vacuum over an extended period of time, and they really cannot withstand high-temperature exposure or solar radiation exposure without serious degradation and outgassing. Metal envelopes with welded seams will hold the required vacuum, but it is virtually impossible to achieve the perfectly leak-free welds required for maintaining very high-grade vacuums over many years, when such welds have to be made in the presence of the billions of microscopically fine glass fibers and perlite particles used in prior art panels. A single particle or fiber intruding into the weld area could create a microscopic leak that would be very difficult to detect, but would nevertheless seriously compromise the lifetime of the vacuum inside the sealed insulation panel, thus compromising the usefulness of the panel.
The use of a vacuum results in the need for a sufficient structure inside the panel to hold the opposite panel walls from collapsing together. The glass fiber mats and perlite powders used in the prior art panels described above can serve that function. However, in high-order vacuums the inwardly directed sidewall pressures become very great so that such fiber mats and powders become more tightly compacted, thus offering more direct heat conduction paths through the insulation panel than desired. Also, to be really adaptable for a wide variety of uses, the insulation panel should be bendable around curves. However, bending the prior art panels would almost certainly crimp one wall sheet of the panel into the other, thereby forming a "cold short" where one wall or sheet touches the other. Even if the glass fiber mats of the prior art would physically hold the two opposite sheets apart, the mat itself would be so compressed at the crimp or bend that it would virtually form the cold short itself.
The laser-sealed vacuum insulating window invented by David K. Benson, one of the joint inventors of this invention, and C. Edwin Tracy, now U.S. Pat. No. 4,683,154, solved the problem of long-term sealing and structural support against collapse or cold short by laser-welding glass spacer beads between two glass sheets. However, that structure is quite thick, heavy, and fragile, being made with glass, and it is rigid, so it cannot be bent around curves. Therefore, while it is a highly efficient insulation panel, its utility is limited.